The invention relates to a sprite-based encoder (encoder and decoder that automatically builds a sprite (also called a mosaic) while operating in a separate shape/texture coding environment such as MPEG-4 and novel applications that utilize the mosaic.
A mosaic image (the term's mosaic and sprite are used interchangeably) is built from images of a certain scene object over several video frames. For instance, a mosaic of a background scene in the case of a panning camera will result in a panoramic image of the background. Two major types of sprites and sprite-based coding are defined in MPEG-4. The first type of mosaic is called an off-line static sprite. An off-line static sprite is a panoramic image which is used to produce a sequence of snapshots of the same video object (such as background). Each individual snapshot is generated by simply warping portions of the mosaic content and copying it to the video buffer where the current video frame is being reconstructed. Static sprites are built off-line and are transmitted as side information.
The second type of mosaic is called an on-line dynamic sprite. On-line dynamic sprites are used in predictive coding of a video object. A prediction of each snapshot of the video object in a sequence is obtained by warping a section of the dynamic sprite. The residual signal is coded and used to update the mosaic in the encoder and the decoder concurrently. The content of a dynamic mosaic may be constantly updated to include the latest video object information. As opposed to static sprites, dynamic sprites are built on line simultaneously in the encoder and decoder. Consequently, no additional information needs to be transmitted besides mosaic dimensions, a blending factor and the global motion parameters.
Both off-line static and on-line dynamic sprite-based coding require constructing a sprite. In the former case, the sprite is built prior to transmission. In the later case, the sprite is built on-line during the transmission. So far, MPEG-4 has assumed that the outline (segmentation) of the object for which the sprite is going to be built is known a-priori at every time instant. Although this is true in certain applications, especially in post-production or content generation using blue screen techniques, automatic segmentation does not currently exist for sprites built dynamically in encoding environments.
Therefore a need remains for sprite-based coding systems where sprite building does not require a-priori knowledge of scene segmentation.